The present invention relates to a lean burn control method and device for an internal combustion engine, and a fuel injection quantity control method and device including the lean burn control method and device. In particular, the present invention relates to a lean burn control method and device for an internal combustion engine to be controlled so that lean burn is performed at the middle point between a theoretical air-fuel ratio and a lean burn limit, and a fuel injection quantity control method and device including such a lean burn control method and device.
As a control method for lean burn, there has conventionally been considered two methods. One of the two methods is a method using a sensor called a wide-range O.sub.2 sensor, which can generate a detection signal proportional to an oxygen concentration. The other method is a method such that it is decided whether or not an air-fuel ratio has entered a roughness (rotation fluctuation) zone, and that a fuel quantity is increased if the air-fuel ratio has entered the roughness zone.
The method using the wide-range O.sub.2 sensor requires an expensive O.sub.2 sensor to cause an unavoidable increase in cost.
In general, it is known that an air-fuel ratio zone where a NO.sub.x catalyst works most is present at the middle position between a theoretical air-fuel ratio and a roughness air-fuel ratio zone, and that the rate of purification of the NO.sub.x catalyst decreases in the vicinity of the roughness air-fuel ratio zone (see FIG. 3). That is, in the method such that burning is carried out until the air-fuel ratio has just entered the roughness zone, and that a fuel quantity is somewhat increased to restore the air-fuel ratio (reduce the air-fuel ratio), so as to improve the burning, there occurs a problem that an emission quantity of an exhaust gas such as NO.sub.x increases.
Further, in an air-fuel ratio control device for a multicylinder engine for controlling an air-fuel ratio of an air-fuel mixture to be supplied to each cylinder to a roughness tolerance limit on the lean side according to an output from burn condition detecting means for detecting a burn condition in each cylinder, it is known to provide acceleration detecting means for detecting acceleration of the engine and control means for controlling a fuel supply quantity at acceleration of the engine according to an output from the acceleration detecting means in such a manner that the smaller the roughness tolerance limit on the lean side in each cylinder, the more the fuel supply quantity is increased (e.g., Japanese Patent Laid-open Publication No. 61-229936).
In this case, however, it is considered that an optimum lean burn air-fuel ratio is present at the middle point between a theoretical air-fuel ratio and a lean burn limit (i.e., the roughness tolerance limit mentioned in the above prior art) from the two viewpoints of emission of an exhaust gas (especially, NO.sub.x) from the internal combustion engine and stable lean burn of the internal combustion engine. Accordingly, the increase in the fuel quantity from the roughness tolerance limit to the fuel-rich air-fuel ratio according to only the output from the burn condition detecting means cause a deterioration in emission.